docs: def stats_per_key(self):
          """
          Return statistics calculated for each key in the container.
  
          Note:
              The feature container has to be opened in advance.
  
          Returns:
              dict: A dictionary containing a DataStats object for each key.
          """
          self.raise_error_if_not_open()
  
          all_stats = {}
  
          for key, data in self._file.items():
              data = data[()]
              all_stats[key] = stats.DataStats(float(np.mean(data)),
                                               float(np.var(data)),
                                               np.min(data),
                                               np.max(data),
                                               data.size)
  
          return all_stats